The objective of the present study was to investigate the protective effect of ulinastatin on severe pulmonary infection under immunosuppression, and its molecular mechanism. Mice were treated with methylprednisolone and lipopolysaccharide (LPS) to establish the model of severe pulmonary infection under immunosuppression. Mice were randomly divided into group A (model group; treated with equal volumes of saline), group B (treated with 1x105 U/kg ulinastatin), and group C (normal control group). Bronchoalveolar lavage fluid (BALF) was collected, and the concentrations of cytokines in BALF were measured by enzyme-linked immunosorbent assay (ELISA). Pathological changes in lung tissues were observed by hematoxylin and eosin (H&E) staining. The mRNA levels of M1 and M2 macrophage markers in lung tissues were detected by real-time polymerase chain reaction (PCR). Specific protein levels in lung tissues were measured by western blotting. Apoptosis in lung tissues was detected by the terminal-deoxynucleoitidyl transferase mediated nick end-labeling (TUNEL) method. The concentrations of TNF-α, IL-6, and IL-1β in BALF, the mRNA levels of the three M1 macrophage markers, and the protein levels of p-Janus Kinase 2 (p-JAK2), p-signal transducer and activator of transcription-3 (p-STAT-3), cleaved caspase-9, and cleaved poly-ADP-ribose polymerase (PARP), and the number of apoptotic cells in lung tissues in group A were significantly higher than those in groups B and C (P<0.05), whereas the concentrations of IL-4, IL-10, and IL-13 and the mRNA levels of the three M2 macrophage markers were significantly lower than those in groups B and C (P<0.05). Immunofluorescence showed that the nuclei of lung epithelial macrophages in group A became smaller and moved towards the side of nuclear membranes. In conclusion, ulinastatin can improve the inflammatory response caused by severe infection under immunosuppression, which balances the inflammatory microenvironment and inhibits apoptosis at least partially through inhibiting JAK2/STAT-3 and/or caspase pathway activity, ultimately playing a role in lung protection.

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